Two-way radio communication systems that support hand-off are well known in the art. An example of such a system is a cellular mobile telephone system. Other systems such as those based on the Digital European Cordless Telecommunications (DECT) standard have been proposed and are under development by equipment vendors around the world.
Conventional analog radio communication systems that have a plurality of radio coverage areas defined by a limited transmission distance of a fixed communication unit in each of the radio coverage areas use a controller to support hand-off. Whenever a need for handing-off a first portable communication unit is determined by a first fixed communication unit handling a call with the first portable communication unit, the controller commands other fixed communication units near the first fixed communication unit to measure the received signal strength of the first portable communication unit on a transmission channel on which the first portable communication unit is transmitting. To avoid a possibility of erroneously measuring a signal transmitted from a second portable communication unit, the identity of the portable communication unit transmitting the received signal also must be determined.
Digital communication systems based on second generation cordless telephony (CT2) technology are becoming popular. A problem exists in executing a hand-off in the conventional CT2 system, because only a fixed communication unit that has established a link with a portable communication unit can identify the portable communication unit. Other fixed communication units can measure the strength of a signal present on a channel that the portable communication unit is known to be using, but cannot confirm that the signal is coming from the "correct" portable communication unit. Worse yet, because the CT2 system is a time-division-duplex system, it is equally likely that a monitored signal may be coming from another fixed communication unit.
The cause of the identification problem is that in the conventional CT2 system there is no frame synchronization information transmitted while transmitting user data, e.g., voice communications. As a consequence, there is no way for a monitoring fixed communication unit to synchronize with and identify the source of a monitored signal. A fixed communication unit that originally establishes a link with a portable communication unit does not have a synchronization problem, because by maintaining bit synchronization with the fixed-bit-rate signals of CT2, the fixed communication unit also maintains frame synchronization obtained during link establishment.
Even if the source of the monitored signal could be positively identified, a seamless hand-off would not be possible in the conventional CT2 system. This is because the conventional CT2 system provides no way of establishing a new wireless link for accepting the hand-off without noticeably interrupting the transmission of user communications.
Thus, what is needed is a way of performing a hand-off in a CT2 system that allows confirmation of the source of a signal while monitoring the signal to select the best fixed communication unit for receiving the hand-off. Also needed is a way of doing a seamless hand-off. Whenever a seamless hand-off is not possible, then at least a way of doing a hand-off that reduces interruption of user communications to a negligibly brief duration is needed.